There is a need to provide various paper grades, including printing and writing quality papers, to be produced with greater brightness. This requires that the pulp furnish, which is needed to manufacture the paper products, have very high brightness.
The bleached chemi-thermo-mechanical pulp (BCTMP), also known as high-yield pulp (HYP) has many unique properties such as high bulk and light scattering coefficient that are desirable for printing and writing paper products [1-5]. However, based on currently available bleaching technologies, the economical brightness levels of HYP are still significantly lower and its yellowness (b*) higher than those of fully bleached kraft pulps (BKP) [1-2]. For example, Aspen HYP can be bleached to about 85% ISO brightness from the alkaline peroxide bleaching process, however, the production cost increases exponentially when a very high brightness (e.g. higher than 80% ISO) is required. Also, in some commercial applications extremely high brightness (higher than 88% ISO) is required, and the conventional peroxide bleaching in these cases would become prohibitively expensive.
Optical brightening agents (OBAs) or fluorescent whitening agents (FWAs) are commonly used to improve the optical properties of printing and writing paper during the papermaking process [6]. Both the paper brightness and whiteness can be increased with the addition of OBA. Also, OBA can decrease the photo-yellowing (photo-reversion) of HYP by acting as an UV screen [7, 8, 9]. It has also been reported that the overall environmental impact of using OBA might be better than peroxide for a certain level of brightness gain of mechanical pulps [9].
Conventionally, OBA is applied to pulp at the wet-end of the papermaking process. OBA molecules adsorb on pulp fibers by forming hydrogen bonding with cellulose in fibers. When OBA is added to a mixed furnish, HYP will compete with BKP for OBA. Ideally, OBA adsorbed on HYP fibers would be more effective in whitening HYP-containing paper sheets. To have more OBA on HYP fibers rather than on BKP fibers, OBA may be added to HYP at the pulp mill during the HYP production process. Another factor affecting OBA efficiency is its retention on fibers. It has been reported that OBA retention decreased with the increase of HYP substitution rate due to the lower affinity of OBA molecules to HYP fibers [10, 11]. As a result, the OBA efficiency decreased. Although the white water circulation system can improve the overall retention of OBA, the un-absorbed OBA molecules in the white water can undergoes a transformation from trans- to cis-configuration and loses its fluorescent properties [6].
U.S. Pat. No. 5,902,454 and European patent application Serial No. EP 0899 373 A1 disclose the use of OBA to increase the brightness of HYP.
Typical state of the art processes for peroxide bleaching of HYP are described in the following references. First Presley, J. R. and Hill, R. T., Pulp Bleaching: Principles and Practice, Edited by C. W. Dence and D. W. Reeve, Page 480 discloses the so-called cascade system for preparation of bleach liquor, in which magnesium sulfate and sodium silicate are added to water and intimately mixed, followed by the addition of caustic soda and finally by the addition of hydrogen peroxide. The resulting liquor is subsequently mixed with pulp.
Presley, J. R. and Hill, R. T., Pulp Bleaching: Principles and Practice, Edited by C. W. Dence and D. W. Reeve, Page 481 discloses the so-called in-line system, where similar mixing and addition occurs but without the cascade arrangement.
Ni, Y. et al., Pulp and Paper Canada, 104(12):78 (2003) discloses a sequential addition of chemicals beginning with the addition of silicate or DTPA, or other peroxide stabilizers to a pulp slurry and the subsequent addition of hydrogen peroxide. Alkali sources, such as sodium hydroxide can be added simultaneously with the stabilizers. The following patents deal with bleaching processes for mechanical or high-yield pulp, but do not address the process of the present invention: Canadian Patent Nos. 686, 115; 820,190; 1,294,655; 1,310,797; 2,041,588; 2,070556.
Therefore, there is a need to increase the brightness in a more cost-effective manner. Also, there is a need to produce HYP pulps with extremely high brightness in a more economic way.